Method for melting refractory metal



Nov. 4, 1969 L. a. STARK ET AL 3,476,171

METHOD FOR MELTING REFRACTORY METAL Filed Feb. 14. 1967 SALT COAT/N6 0 8/05 WALL 0F CRUC/BLE UNCOATED BOTTOM WALL 0F CRUC/BLE INVE/V TORS. LOU/.5 E. STARK 8 OCTAVIA/V BERTEA A I torney 3,476,171 METHOD FOR MELTING REFRACTORY METAL Louis E. .Stark, Youngstown, and Octavian Bertea, Warren, Ohio, assignors to Reactive Metals, Inc., a corporation of Delaware atent O 3,476,171 Patented Nov. 4, 1969 halide and a liquid having a low vapor pressure and substantially no solubility for the halide in such a manner as to leave the bottom 13b uncoated to carry the electric current necessary for the consumable arc melting process. The coating is dried before melting to completely Filed Feb. 14, 1967, Ser. No. 615,929 5 remove the liquid, in any suitable manner such as by Int. Cl. 1522c 3/00; B22d 27/02 applying a vacuum so that the liquid vehicle is removed CL 6 Clalms by evaporation. After the coating is dried, the crucible 13 may be used for melting. The are 15 used for melt- 10 ing is prevented from striking the crucible side wall 13a ABSTRACT OF THE DISCLOSURE by the presence of the electrically non-conducting coat- An improvement in the consumable arc melting of g 14, thus making the melting process more efiicient refractory metals such as titanium or zirconium which by transferring all of the are heat into the ingot 12 involves collecting the molten refractory metal in a cooled Metal ingots produced in accordance with the invention crucible having a coating at least 0.0 l-inch thick on the 15 by C ns mable arc melting have a smooth improved interior thereof of at least one halide from the group Surface- During melting, the ingot is in Contact With the consisting of halides of alkali and alkaline earth metals. C ating On the Crucible side wall and, as a result, the heat of the solidifying metal is conducted more slowly than it would be if in direct contact with the copper crucible. The This invention relates to a method of producing ingots retarded cooling thus allows the solidified metal to coof refractory metals such as titanium and zirconium. alesce in a smooth dense outer wall. Moreover, because More particularly, the invention concerns an improveof the slower heat conduction from the molten metal ment in a method of producing ingots by a consumable to the crucible, the internal solidification of the ingot arc melting process whereby the ingots possess superior can proceed more uniformly with less tendency for inwall surfaces and minimum internal porosity. ternal porosity by solidification shrinkage. The greater In processing certain refractory metals, the refining uniformity of internal solidification results in an ingot process is performed in a consumable arc furnace wheref Sounder internal metallurgical Structurein molten metal is collected in a crucible which acts as After completion of the metal refining process, the a portion of the electrical circuit. In a typical applicaingot is cooled and subsequently extracted from the crution, such as in titanium refining, a water-cooled copper cible with considerable ease because of the reduced friccrucible is used. Contact of the molten metal against the tion between ingot and crucible resulting from the prescooled surface is responsible for the formation of a spongy ence of the intermediate coating material. The crucible and irregular ingot wall surface caused by non-uniform maintains its original surface undamaged by arcing or solidification of molten metal. There is also occasional galling during ingot removal and consequently its useful arcing to the crucible which causes surface pitting and life is materially improved. melting of the copper container. The pitting and defor- In summary, therefore, the benefits resulting from the mation of the copper crucible renders the ingots difficut use of coated crucibles in accordance with the invention to remove after solidification because of the non-uniin consumable arc melting include (1) more efiicient form wall surface and the contact between the ingot and transfer of are heat into the ingot because of the presence crucible. 40 of the electrically non-conducting coating on the crucible; In the drawing: (2) greater ease of ingot extraction due to the reduced The single figure is a diagrammatic vertical sectional friction and increased space between ingot and crucible view of a conventional consumable electrode furnace to resulting from the presence of the coating; (3) increased which our invention is applied. crucible life by elimination of erosion or burn-through The present invention provides a method of casting from the are striking the crucible wall and from reduced ingots of refractory metal such as titanium and zirconisurface galling during ingot removal; (4) improved um which avoids the aforementioned difficulties. Accordsmoothness and soundness for the outer wall of the ingot ing to the invention, consumable arc melted refractory because of more favorable characteristics for metal solid metal of superior homogeneity to the original electrode ification in contact with the salt coating; and (5) im- 10 is collected as an ing t 12 within a crucible 13 which proved internal metallurgical, mechanical, and chemical has a thin adherent and continuous coating 14 at least characteristics of the ingot due to more uniform internal 0.0l-inch thick, preferably 0.06-inch thick and not greatsolidification and cooling characteristics resulting from er than about 0.25-inch thick, of at least one halide of an heat conduction through the coating. alkali or alkaline earth metal. The coating is preferably The following will illustrate the practice of the inapplied to the crucible side wall 13a as a slurry of the vention.

TABLE I Crucible Crucible Coating diameter Melting (in.) proe. Salt Vehicle Remarks Sample: 4 V N I 1 4. 5 V ens lngo gg lflieult to remove, coating spongy.

6 P .do Do. 4. 5 V 50% KC150% NaCl Methanol.-. Ingot easily removed, good surface.

6 P 50% KCl-50% NaCl 0 Do. 4.5 v CaF Do. 6 V CttF Do. 4. 5 V NaCl do Ingot difficult to remove, coating spongy! 6 P NaCl do Ingot easily removed, good surface. 4.5 V NaCL- Water Do. 6 V NaCL. do Do. 8 V NaCl. do. Do.

1 V denotes vacuum melted. P denotes melted under positive pressure of Argon.

A series of ingots were melted in crucibles having coatings as described in Table I. The coatings were applied by brushing a slurry of the indicated salt in a suitable vehicle such as methyl alcohol or water. The salt was mixed in proportions sulficient to produce a heavy paste easily applied to the crucible wall with a brush to form an adherent, continuous coating surface. In the examples illustrated, a proportion of about 100 grams of salt to about 80 ml. of liquid vehicle produced a suitable paste for application to the crucible side walls.

It is necessary that the coating be applied to the crucible wall in suflicient thickness to achieve the desired results of the invention. We have found that a thickness coating of at least about 0.01-inch is necessary to insure sutficient retardation of the cooling of the ingot and ease of the ingot removal from the crucible. In Example No. 8 of Table I, a coating less than about 0.01-inch thick was applied and as a result, the ingot was difficult to remove and had a surface not altogether satisfactory.

It has also been shown that an aqueous coating mixture may be used satisfactorily. However, the mixtures made with water have less tendency to adhere and the water does not dry as quickly as the organic vehicles and, therefore, such coatings require a longer time to dry. The coating may be applied to the crucible in any convenient manner so long as it is present as a continuous adherent coating on the crucible side wall and has a minimum thickness, as discussed above, of at least 0.01-inch.

We claim:

1. In a process for refining a refractory metal, which process includes suspending a consumable electrode of the refractory metal over a metal crucible, Water cooling the crucible, and applying an electrical current through the electrode and crucible to produce an are at the bottom of the electrode, thereby melting the electrode and building up an ingot in the crucible, a method of improving the surface characteristics of the ingot, which method comprises applying to the inside surface of the side walls of the crucible exclusive of the bottom wall an adherent and continuous coating about 0.01 to 0.25- inch thick of a salt of the group consisting of halides of alkali metals and alkaline earth metals, said coating being applied to the crucible walls as a paste and dried before melting of the electrode begins, and conducting the arc current through the uncoated bottom wall of the crucible to the ingot during the melting operation.

2. An improvement according to claim 1 wherein said refractory metal is titanium.

3. An improvement according to claim 1 wherein said salt is sodium chloride.

4. An improvement according to claim 1 wherein said salt is calcium fluoride.

5. An improvement according to claim 1 wherein said crucible is a cooled copper mold.

6. An improvement according to claim 1 wherein said coating is about 0.06-inch thick.

References Cited UNITED STATES PATENTS 1,940,619 12/1933 Barstow et al. 75-17 2,607,674 8/1952 Winter 75-84 3,035,318 5/ 1962 Campbell. 3,271,828 9/1966 Shelton et a1. 164-52 3,417,808 12/1968 Rosenberg 164-68 FOREIGN PATENTS 808,843 1/1959 Great Britain.

OTHER REFERENCES Slag Washing of Stainless Steels Steel, May

25, 1959, pp. 146, 149, published by Penton.

J. SPENCER OVERHOLSER, Primary Examiner V. RISING, Assistant Examiner US. Cl. X.R. 

